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A neat looking design indeed. I have seen only a couple built - and just one channel at a time as it is hard to find a suitable output transformer . As noted, the choice of output transformer will be very important in this circuit. The article says 6k with a DC resistance of 250 ohms on the secondary. Look for a transformer with a primary of 5 to 6k, but you have to stay pretty close to a DCR of 250 ohms on the secondary.

If the DCR is not high enough a fixed resistor could be put in series with the primary. It would waste a little output but not really make that much difference. You could if desired use a fairly large value capacitor across the resistor and that should restore nearly all the output.

Sorry to dig this out of the grave, but I have the same question : does anybody have built this amplifier ? I have a pile of 6SJ7, a pair of 5k SE OPT, and pretty much everything else that’s needed. I’m curious about the direct coupled topology.

I would measure the resistance of the OT's, they may be closer to the 250 ohm specified. I would try it even if they only measured 230 ohms (because 230 / 250 is .92 or 92 % of the value specified. You could place a resistor in series with the OT to come to the value of 250 ohms , maybe a power resistor.

As you can see in the schematic description, both impedance and resistance are specified. I assume that since the transformer replaces the cathode resistor, it must have a precise resistance (dc).

Not quite.

The 6V6 cathode load is the series combination of R3 (1000Ω), R5 (355Ω), and the transformer primary dc resistance (250Ω) for a total of ≈1.6kΩ. This (along with the B+ voltage) sets the 6V6 quiescent plate voltage but NOT the grid bias. The grid bias is set by the resistive voltage divider formed by R3/R5/R6/R7. The 49.5mA cathode current drops 12.4 volts in the transformer primary which adds to the voltage set by the R6/R7 divider to set the cathode at 85v against the grid at 73v. Total grid bias is therefore -12.4v. (i.e. essentially -12.5v, the most common 6V6 grid bias.) Also note that the plate voltage works out to be 355-85=250v, the most common plate voltage for the 6V6 tube.

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